1. Field of the Invention
The present invention relates to a coated hardmetal body comprised of a basic hardmetal body, or core, containing cobalt and tungsten carbide and of a hard substance layer free of binder metal, with the hard substance layer being comprised of hard oxides, a mixture of hard oxides and hard nitrides, or of a mixed crystal of hard oxides and hard nitrides. The present invention further relates to a process for producing the coated hardmetal body herein described.
2. Description of the Related Art
German Patent No. 2,233,699 discloses hardmetal parts which are provided with a coating having a thickness of up to 50 microns of at least one hard, heat resistant compound. The heat resistant compound employed may be a hard oxide, particularly magnesium oxide, hafnium oxide, chromium (III) oxide and/or aluminum oxide, a hard nitride, particularly silicon nitride, vanadium nitride, niobium nitride, aluminum nitride, boron nitride, titanium nitride, zirconium nitride, tantalum nitride and/or hafnium nitride, or a hard boride, particularly titanium diboride, tantalum diboride or hafnium diboride. According to German Patent No. 2,233,699, a coating for a hardmetal body may also be comprised of mixtures or mixed crystals of the above-mentioned hard substances. The hardmetal basic body, or core, of the coated hardmetal body is made of a hardmetal which is composed of a binder metal and of tungsten carbide, titanium carbide, tantalum carbide and/or niobium carbide.
It has been found that the oxygen containing hard substance layers, particularly hard substance layers made of oxides, do not sufficiently adhere to the hardmetal core and chip off under comparatively slight stress. Frequent attempts have been made, therefore, to improve the adhesion of the oxygen containing hard substance layers to the hardmetal core.
Thus, U.S. Pat. No. 3,837,896 discloses a cutting bit made of a hardmetal core, an intermediate layer and a surface layer free of binder metals made of one or a plurality of extremely wear resistant precipitates of aluminum oxide and/or zirconium oxide, wherein the intermediate layer is composed of one or a plurality of carbides and/or nitrides of the elements titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, silicon and/or boron and is free of binder metals. The binder metal free hard substance intermediate layer composed of carbides and/or nitrides provides a good bond between the oxide surface layer and the hardmetal basic body.
U.S. Pat. No. 4,019,873 discloses a wear resistant shaped body which is comprised of a hardmetal basic body, an intermediate layer of one or a plurality of borides and an outer layer of aluminum oxide and/or zirconium oxide. The boride intermediate layer, which is composed, in particular, of the diborides of the elements titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum and tungsten, is likewise intended to improve the adhesion of the extremely wear resistant oxide outer hard substance layer.
Finally, U.S. Pat. No. 3,261,673 discloses the application, on a tungsten carbide hardmetal body, of an intermediate nickel layer on top of which an aluminum oxide layer is sprayed, with the nickel intermediate layer providing better adhesion for the aluminum oxide layer and improved heat and corrosion resistance for the compound body.